Add notification channels and a provider abstraction

Users get a notification_channels table (replacing users.ntfy_topic); pages fan
out to every enabled channel through a notify.Provider registry, with ntfy as
the first provider. Fallbacks are typed {type, address}. Schema edited in place:
existing databases must be recreated.

Claude-Session: https://claude.ai/code/session_016mBLURvJoMuUEr9cB2RpUN

internal/api/members_test.go

@@ -40,23 +40,23 @@ func TestMembers_StatusReflectsRotaAndPageability(t *testing.T) {
 	s, _ := notifyTS(t, testNotify()) // admin is on call today, with a topic
 	teamUser(t, s, "reachable", "terdut-reachable")
 	silent := teamUser(t, s, "silent", "terdut-silent")
-	s.exec(t, "UPDATE users SET ntfy_topic = NULL WHERE id = $1", silent)
+	s.exec(t, "DELETE FROM notification_channels WHERE user_id = $1", silent)
 
 	got := readMembers(t, s)
 	if m := got["admin"]; m.Status != "oncall" || !m.OnCall || !m.Pageable {
 		t.Errorf("the person on call should read on call, got %+v", m)
 	}
 	if m := got["reachable"]; m.Status != "reachable" || m.OnCall {
-		t.Errorf("a member with a topic who is off the rota is reachable, got %+v", m)
+		t.Errorf("a member with a channel who is off the rota is reachable, got %+v", m)
 	}
-	if m := got["silent"]; m.Status != "unpageable" || m.Problem != "has no ntfy topic" {
-		t.Errorf("no topic means they cannot be paged, got %+v", m)
+	if m := got["silent"]; m.Status != "unpageable" || m.Problem != "has no notification channel" {
+		t.Errorf("no channel means they cannot be paged, got %+v", m)
 	}
 
 	// Being on call does not rescue an account that cannot be woken.
-	s.exec(t, "UPDATE users SET ntfy_topic = NULL WHERE username = 'admin'")
+	s.exec(t, "DELETE FROM notification_channels WHERE user_id = (SELECT id FROM users WHERE username = 'admin')")
 	if m := readMembers(t, s)["admin"]; m.Status != "unpageable" || !m.OnCall {
-		t.Errorf("an on-call person with no topic is the red case, got %+v", m)
+		t.Errorf("an on-call person with no channel is the red case, got %+v", m)
 	}
 
 	s.exec(t, "UPDATE users SET disabled_at = 1 WHERE id = $1", silent)